The activity of dopamine cells of the ventral tegmental area (VTA) plays a critical role in reward and motivation. In vivo, these cells fire irregularly, with interspersed burst events; this pattern of activity is the consequence of excitatory and inhibitory inputs arising locally and from distal structures. In addition, plasticity of these inputs occurs after exposure and withdrawal from addictive drugs. However, understanding the functional role of specific afferents to the VTA, both in baseline conditions and after exposure to drugs, has been limited by current technology. These studies will establish how the activity of the VTA is modulated by specific afferents, before or after cocaine self-administration in rats. To dissect the role of specific afferents, we will use the technique of optogenetics whereby the activity of selective pathways can be increased with pulses of light applied to the terminals that express the algae protein channelrhodopsin 2 (ChR2). Specific brain regions of adult rats will be infected with an adeno-associated virus for ChR2 expression in neurons. We will then determine the functional role of specific pathways by applying light pulses in the VTA, to stimulate the afferents from each of these regions. We will examine inputs to the VTA from (i) the pedunculopontine tegmental nucleus (PPTg), a mixed glutamatergic/cholinergic population that responds to salient stimuli; (ii) local VTA glutamate cells, which have recently been described but whose functional role is unclear; and (iii) the rostromedial tegmental nucleus (RMTg), a hindbrain structure recently identified that sends important GABAergic projections to the VTA but whose functional role is unknown. We will evaluate the consequence of pathway-specific stimulation on cell activity (firing rates and patterns) measured in vivo in anesthetized rats. Then, we will determine the nature of these inputs in vitro, by measuring synaptic potentials generated upon stimulation of these pathways. We will also determine the manner in which such synaptic input is integrated. Finally, we will further test the nature of the pathways by performing immunohistochemistry studies. Aim 1 will examine the role of these pathways in drug-naove rats. Aim 2 will examine it in rats that have self-administered cocaine. These studies will be the first to determine the functional role of unexplored and novel afferents to the VTA. They will also determine how cocaine self-administration modifies the way dopamine cells are excited by specific afferents. This will provide important information on the manner in which brain reward pathways function in baseline conditions, and their plasticity after self-administration. These studies will provide novel insights on neural circuits that control addictive behavior.